1. Technical Field of the Invention
The present invention generally relates to induction heating equipment. More particularly, and not by way of any limitation, the present invention is directed to a portable induction heating system utilizing a power generator that is capable of outputting high frequency signals at low impedances.
2. Description of Related Art
The principles underlying the phenomenon of induction heating, i.e., the process of increasing temperature in a conductive component (referred to as a susceptor) by coupling to a magnetic flux generated from an inductive coil, are well known. It is also well known that equipment based on the principles of induction heating is deployed in various fields.
Several deficiencies and drawbacks are extant in the current inductive heating solutions, however. Traditional switching power supply (SPS) systems used for sourcing power in conventional inductive heating equipment are bulky because of the size of the various components employed therein. Also, such SPS systems are typically provided to be operable to drive a relatively higher ohmic resistance, e.g., 50 xcexa9 or so. Accordingly, large step-down transformers are used for matching output to lower impedance loads in order to generate the high current, high frequency signals needed for creating appropriate levels of magnetic coupling. Consequently, the existing induction heating systems are generally not readily amenable to portability.
Further, the operating frequency ranges of today""s induction heating systems are not effective in certain important applications. For example, whereas frequency ranges of around 100 KHz generated by the current equipment are suitable for applications such as automotive industry (e.g., selective or localized hardening of crank shafts by rapid annealing and quenching), such frequencies are not useful for coupling to small conductive elements such as, e.g., metallic interconnects in integrated circuits, bonding pads, solder joints, bumps or balls used for attaching multiple semiconductor die together or to various other substrates, or other components having portions with limited effective surface areas that require rapid heat treatment. It is known, however, that substantially higher frequency ranges (in the MHz range) are needed to couple to such tiny susceptors effectively.
In addition, where sufficiently high frequencies are generated, power supplies of such induction systems are typically tuned to and fixed at a particular frequency for achieving resonance. As a consequence, these induction heating systems are not suitable for susceptors of different sizes and shapes because of the resultant changes in resonance frequency which drastically reduce the AC current delivered to the induction coil and create undesirable feedback to the power supply.
Accordingly, the present invention provides a portable induction heating system which utilizes a broadband high frequency high-power, low output impedance power generator formed from switching MOSFET devices for directly coupling to low impedance inductive coil loads. In the presently preferred exemplary embodiment, the power generator circuit is designed to drive extremely low impedance loads on the order of a fraction of an ohm, thereby advantageously avoiding the need to match the power output of a conventional switching power supply by incorporating bulk step-down transformer systems. Accordingly, the power generator is highly compact and portable, weighing in under 10 pounds, and yet capable of generating high power signals from 0.5 MHz to 20 MHz or greater.
A voltage-controlled oscillator (VCO) or any microprocessor-controlled signal generator drives a power output stage under feedback control so as to effectuate resonance at a high frequency in an induction coil assembly (i.e., load) connected to the power generator. The power generator is operable with a switching regulator that can supply a fixed DC voltage, e.g., from around 10 V to about 50 V. The induction coil assembly (which includes a capacitive circuit portion connected to a conductive coil) couples a magnetic field to a susceptor component for inductively heating it.
A microcontroller is provided for inputting operating parameters such as power, frequency, duty cycle and duration, and is operable to auto-tune the VCO output by sweeping frequency at startup as well as by controlling drift during operation under a changing load. Sensors such as, e.g., temperature, current, phase, and voltage sensors are used to provide feedback to the microcontroller. Preferably, the sensor feedback is used to control power delivered to the load as a function of changing load impedance via changes in frequency, signal duration, and DC voltage.
In one exemplary embodiment of the present invention, the power output stage is comprised of a plurality of buffer pre-driver circuit portions. Each pre-driver portion is capacitively coupled to a driver circuit portion comprising at least one transformer whose secondary coil is operable to drive the gate of an NMOS device. The output of the driver circuit portion is staged through a plurality of NMOS devices organized into two banks for alternately switching output power to the load via a connector.
In another exemplary embodiment, the power output stage includes a pure MOSFET design, wherein the use of transformers in the pre-driver circuitry is eliminated. The driver circuitry in this embodiment is preferably comprised of NMOS and PMOS devices in a CMOS circuit arrangement with appropriate DC offsets provided for the gates. The output of the driver circuitry is staged through a bank of NMOS devices and a bank of PMOS devices for driving the low impedance coil assembly load.
A variety of coil configurations can be provided for use with the induction heating system provided in accordance with the teachings of the present invention. Because the series resonant coil and the capacitive circuit portion coupled thereto are preferably provided as a module external to the power generator, appropriate capacitor/coil combinations may be had for target-specific applications.